Nghiên cứu băng tần 300khz đến 30 MHz

IEEE TRANSACTIONS ON COMMUNICATION TECHNOLOGY VOL I n addition to the device reliability considerations, an environmental study was made to verify that the system could withstand shipping, storage, and operating conditions on the customer's premises Laboratory tests which included temperature shock, high relative humidity, and vibrationwereused to stimulate the expectedenvironmental extremes to which the equipment would be subjected in actual use As a result of these tests minor design modifications were incorporatedprior to initial production The majorityof equipment malfunctions a,re expected to be repairable by merelyinterchangingprintedwiring boards Trouble shooting of common circuitry consists of the analysis of symptoms to narrow the troubledown to a COM-14, NO DECEMBER 1966 small number of possible circuit packs, and then the replacement of theseone at atimeuntilthe defective board is located Troubles on traffic circuits, such as registers and centraloffice trunks, can be isolated by a feature which enables a repairman to route test calls to specific circuits To facilitate testing, t,he built-in test equipment, fuses, and alarms are located at the front of the cabinet, a t eye level CONCLUSION The 800A PBX was introduced into commercial service in August 1966 Operational experience with the system has been very good 300 kHz-30 MHz MF/HF appropriate that it stillbe presented for the edification of those people desiring to understand this area of activity and for the sake of completeness I n order to justice to the broad spectrum covered by this paper, it will be necessary to break apart the 0.3-30 nilHz frequency slot into four categories and then discuss three of these categories (the medium frequencies) in a limited way while reserving the bulk of the discussion to the 4th category 3.0-30 MHz (the high frequencies) The so-called mediumfrequency (MF) spectrum extending from 0.3 toMHz, for the purpose of this presentation,asjuststated, will bedivided intothreedistinct HE PURPOSE of this paper will be to serve as a regions approximated by region A 300-550, kHz, region B, broad tutorial coverage of the elements and factors 550-1650 kHz and region C, 1650 to 3000 kHz Region A, employed for characterizingthevarious channels, as employing CW transmission almost exclusively, generally segmented assignments, inthe frequency spectrum extend- is utilized for navigational purposes, for mobile, aeroing from 300 kHz to 30 MHz In this connection, some of nautical and ship communications, for emergency survival the propertiesof significance such as the temporal behavior communications, and for time and frequency synchronizain termsof signal levels and noise, channel transfer proper- tion Region B is employed for standard broadcast service ties, interference, fine grain behavior, and system performand region C is and may be utilized for fixed and mobile, ance as exemplified by both theory and experimental data, land, maritime and aeronautical navigation, and commuwill be covered nication purposes Liberal usewill be made of material already in the open In these three regions most of the useful distant field literature, material available to USAEL through their varenergy is propagatedbytheground or surfacewave ious contracts with industry and universities, and data and The sky wavegenerally presents a sourceof trouble, howinformationgeneratedasa result of USAEL'sown in- ever, it is occasionally utilized as the primarymode, espehouseprograms cially for region C Although a good portion of the material to be covered As far as the surface wave support is concerned, which will not be new to workers in this field, it is considered can be viewed as due toan earth-atmosphere wave guide, the signal strength is reasonably well behaved Generallyit Manuscript received March 28, 1966; revised August I, 1966 Paper 19CP65-482 presented at the 1965 IEEE Communications follows an inverse distance law with the value of signal Convention, Boulder, Colo strength being a function of the polarization, operating The anthor is with the Communications/ADP Laboratory, frequency,andthegroundconductivityand dielectric U.S Army Electronics Command, Fort R'lonmouth, N J Abstract-A tutorial presentation is made in broadand general terms regarding the properties of the MF and HF portions of the radio spectrum as they pertain to andaffect communication systems The fine grain behavior in terms of amplitude and phase variations are presented in conjunction with the effects of fading periods, time and frequency spread, and atmospheric noise A discussion of both theoretical and experimental bounds in error rate levels of digital systemsas a function of the basic attributes of the ionospheric channel is undertaken in connection with the adaptive approach to communication system design Two adaptive systems are described briefly in terms of their ability to cope with the time variant dispersive ionospheric channel T 767 765 I E E EDECEMBER TRANSACTIONS TECHNOLOGY ON COMMUNICA'IlON constant along the pathof propagation I n this regard, sea made noise background rather than variationsin the signal water (conductivity X lo-" EMU,dielectric constant 80 support mechanism Designs of communication systems in this region of the ESU),provides the pathwith the least attenuation Apoor earth,that is, earthwitha low conductivi.ty EMU) spectrum are basically easily established in terms of reaand low dielectric constant (3-5 ESU) yields a path with sonably well behaved and understood factors.The additive relativelyhighattenuation.There is littlediurnal or disturbances, characteristicsof applicable antenna systems, transmission path loss, etc are comfortably takeninto annual variation in the ground wave characteristic I n region A, under good conditions, ground wave propa- account I n general, Region A tends to berelatively free fromthe gation can reach1000 miles with only40 dB more loss than which would that due to the inverse distance loss Theoretical work of effects of suddenionosphericdisturbances affect the signal support mechanism and is considered a significance in this area has been performed by Sommerfeld, relatively reliable portion of the radio spectrum in this Morton,vanderPolandBrenner,Watson,andWait Sky wave propagation for this group of frequencies exhibits regard Boththephaseandamplitude of groundwave properties which are dependent uponthe stateof the iono- signals tend to be of high stability In thepresence of sky sphere with signals experiencing of 1-2 Hz representingthe dirunal 2, change in level by a fac- wave thereis a phase lag tor of from to as a function of sun spot activity The variations of ionosphere layer heights Region B, the broadcast band, is known to all of us in existence of the sky wave gives rise to fading and interference effectsa t locations where110th the groundwave and terms of the local range of coverage of the various broadcast stations For the entertainment purpose it is intended sky wave are received This interference effect tends to take place with maximum severity at distances of a few to satisfy, there is relatively little one can complain about hundred miles from the transmitterwhere both theground (excluding program material) except during thunderstorm wave and the skywave are of equal strength activity or nighttime This portionof the spectrum can be I n general, sky wave signals experience diurnaland sea- considered as well disciplined and static with itsuse detersonal variations superimposed upon the variations due to mined by very rigid control I n this portion of the band, the sun spot cycle Fortunately, during the daylight hours exceptforanomalouspropagationbehavior and local there is high absorption in the :D region, hence, the sky lightning activity, reception conditions are quite adequate wave tends to beproblem a only during nighttime when the The mode of modulation universally employed, using voice D layer disappears.The impacto:i ionospheric propagation or music signals, is double sideband amplitude modulation on medium frequencies and high frequencies (HF) will be Some activity is underway to try to employ compatible becovered in the detailed discussion of the frequency region single sidebandfor this service Generally, noisy signals come a problem only near the service range fringes where from 3-30 MHz Some pertinent properties of sky wave transmission at although intelligibility may still behigh, esthetic appreciaMF, however, are best cited at this time The envelope of tion factors are quitelow This portion of the band has its the received signalin the majority of cases tends tofollow a problems at nighttime whenthe high absorption properties of the D layer are no longer available to reduce the unRician distribution which could be viewed as the combination of a Rayleigh distribution and a specular component desired sky wave support It has been common experience, especially while traveling in anautomobile, to hear stations The fade rate is roughly 0.01 per second implying long fades Equally rough estimates of the correlation distance from distant points, 1000 miles or more, with clarity and exceed the local station one had been forspaced antennasindicates that about 20 km is re- strength that at times tuned to quired for decorrelation to a value / ~ I n this portion of the spectrum, information bandwidths As alreadystated for the lvIF region, theground of kHz are generally employed with some stations rewaveisgenerally the mostilnportant primarily beceiving authority for 10 kHzinformationbandwidths cause the energy is reasonably constant (nonfading) and Most assignments in this frequency band are on aregional appears compacted as a specular ray It is interesting to basis with joint sharing of frequency coupled with reliance note thatbecause of this specular nonfading characteristic, on geographical separation for noninterference.It is necesdiversity reception would not enhance system reliability sary at nighttime for some stations toreduce their radiated unless it could operate on the presence of uncorrelated noise power or even go off the air in order to minimize the possior interfence In thelower frequelncy portions of this specbility of their creating interference to a distant station trum limitationsdevelopinterms of antenna efficiency when undesired sky wave support would be prevalent with values of 10 percent being considered good and with The last region in the medium frequency range, region a communication bandwidth capa,bilityin therange 100 to 500 cycles being typical Unfortunately, for this portionof C, basically employs ground or surface wave for its propathe radio spectrum (region A) atmospheric noise is quite gation support butis more seriously troubled by the preshigh being roughly about two ord!ers of magnitude greater ence, in most instances,of undesired sky wave propagation than the levelin the highfrequency 3-30 MHzband This portion of the spectrum is highly crowded as is the I n general, communications reliability in this region tends entire range 0.3-30 MHz Although the atmospheric noise to be limited by this noise factor which is further aggralevel is lower in region C than at the region A portion of vated by the ever present and generally increasing man- mediumfrequencyrange, the groundwavedistance at 1966 769 GOLDBERG: MF/HF COMMUNICATION SYSTEMS acceptable attenuation levels is much shorter Generally, this region is used forcommunication distances of up to 100 to 175 miles This frequency region however is subjected to the effects of ionospheric disturbances Basically, thisportion of the spectrum is quite stable in the daytime when the D layer is available to attenuate thehigh angle radiation At nighttime, however, the level of interference due to sky wave support from distant stations makes the situation in this portionof the band something less than desirable Largedirective antenna arrays areemployed by marine operators in order to enhance reliability for communication A large percentage of the traffic carried in this region is amplitude modulated kHz voice signals with possibly the inherent redundancy in unprocessed speech making this segment of the spectrum useful a t night Thisreasonably stable segmentof the MF band is also employed for Loran purposes; however, diurnalvariationsinphaseandamplitude of received signals are evident IONOSPHERIC REFLECTION Throughout thispreceding material it has been indicated that skywavesupportrepresents an undesirable phenomenon In thematerial tofollow, which in essence represents the bulk of this paper, the mechanism of sky wave support, which is essential for communication in the H F range, will be explained and, more importantly,the impact of the resulting effects on communication systems from such support will be covered in detail The ionospheric mechanism provides forwardsupport in the range 3-30 MHz bymeans of specular reflection, refraction, or byscatterwithinthe ionized medium.About 60 years ago, the idea of an ionized layeraboveand concentric with the Earthwas conceived of independently by Iiennelly and Heaviside as a means of explaining the phenomenon of long-distancecommunicationorders of magnitude beyond line of sight distances The upper regions of the Earth’s atmosphere become less dense as one proceeds away from the earth In theregion from approximately 50 km to 450 km, one can find molecules of oxygen, nitrogen, nitric oxide, and rarer gasses in dispersion It is generally believed that ultraviolet radiations and corpuscular bombardment from the sun are the main agents in causing the gasses to ionize in the upper atmosphere The level of this ionizationis not uniform throughout the region from 50 to 450 km, in fact, the ionization is distributed in layers having peak intensities a t particular heights The ability of the ionosphere to provide propagation support is related simply to thecondition that itsrefractive index at radio frequencies is different €rom that at free space A wave incident to the ionospheric layer a t angle will be bent toward the horizontal and then back to Earth with a rate that is dependent upon the electron density and theangle of incidence This phenomenon canbe related to the refractive index as follows: (1) where refractive index of ionospheric medium electron density in electrons per cc e , v a = charge and mass of electron Eo = permittivity of freespace w = radianfrequency u N = = The electromagnetic wave will reach a maximum height prior to returning to Earth at the point where N is large enough to reduce the value of u so that u = sin (2) where 4, as defined previously, is the angle of incidence of the electromagnetic wave withthe ionized layer An important application of the above relations is in their use in obtaining what is known as the critical frequency for the case where the electromagneticwaveis verticallyincident,hence = 0, sin = 0, and u =O Such a wavewill reach a height determinedby N and then be returned to Earth The relationship is as follows: when the proper constants are substituted.I n this case f is the frequency of the wave in MHz This critical frequencyfo obtainedby rearranging (3) N fo = x 104 = x 10-3dR is the highest frequency which can be reflected as a result of vertical incidence It is obviously only dependent upon N , the electron density Soundings by pulse transmission probing with vertical incidence provides a means for determining this critical frequency I t s useisfundamentalin engineering communications circuits and estimating proper operating frequencies b y means of the relationship f(MUF) = fo sec (5) where f(MUF) is designated as the maximum useable frequency foroblique transmission, fo is the critical frequency from vertical sounding, and is the oblique path angle of incidence This relationship is based upon ray theory and neglects the Earth’s magnetic field I t s use for prediction purposes is quite adequate, in view of other assumptions made in theprediction process Figure depicts the phenomenon of refraction resulting from the effect on the velocity of wave front propagation in a mediumof changing refractive index The ionospheremedium has classically beendivided into a number of regions That portion below 90 km is known as the D region; its existence is predominantly a daytime phenomenon The level of ionization is approxi- 770 IEEE TRANSACTIONS ON COMMUNICATlON TECHNOLOGY DECEMBER \ / Fig Refraction of wave Fig Electron density vs height mately lo2 electrons/cc a t 70 k:m, lo3 electrons/cc a t 80 km, and lo4 electrons/cc a t 90 lcm Of necessity, electromagnetic wavesused for long distanceE and F layer propagation pass through this region twice The D region, because of its relatively higher concentration of neutral particles and heavy ions, extracts energy from a passing wave as a result of collisions with electrons excited by the wave As far asH F propagation js concerned, this region is viewed asanattenuationband However, at nighttime when this attenuation is not present,we find phenomenal propagation support for distant transmitters which generally cause a large increasein b,zckground interference The E region is considered as existing from 90 km to approximately 160 km with a maximum region ionization at about 110 km The electron density at this height is in the order of lo4 to lo5 electrons per cc during daylight hours At nighttime there is stillsome ionization, but it is much weaker The critical frequtmcy drops about an order of magnitude from its daytime value I n addition to the normal E layerionization, thereappears occasionally patches of denser ionization a t E: layer heights that seem totravelas ionization clouds Thisunpredictable phenomenon is called Sporadic E and has been responsible for creating interference, because of its superior support for radiated energy, into areas not normally engineered to allow for these signals The E region is useful for propagation supportfor distances up to2000 km, using frequencies as high as 20 MHz The region above 160 km is hewn as theF region This region classically has been dividlxl into layers known as the F1 and Fz layers The F1 layer generally exists during daytime at about a height of 200 km, while the Fz layer exists in theregion 250 to 4.50 knl The F1 layer is notgenerally considered as providing th'e basis for well-engineered long-distance communication The F1 layer merges with the F2 layer a t nighttime to a height of about 300 km The electron densityin thisregion is inthe order of lo6electrons per cc The 300-km layer height, called the F layer, is usually considered as the basis for circuit engineering The use of single hop transmission, becauseof the much greater height of the F layer, can provide support t o a distance of 4000 km or more Frequencies as high as 50 MHz (when the ionization level is high) canbe utilized forthis mode Figure shows a profile of the three regions and depicts the electron density For various reasons, including its high absorption and low electron density, the D region has not been fully examined because of instrumentation difficulties However, it is known that the D layer electron density varies with the 11-year solar sun spot cycle and with the sun's zenith distance The electron density in thisregion is a maximum a t noon and during the summer The E layer is generally well behaved except for the unpredictableappearance of Sporadic E The electron density just before dawn rises from a low value a t night to a maximuma t noon then begins to fall againto a low value after sunset The E layer ionization does not change much as a function of sun spot activity, nor does it vary much with changes in season The critical frequency of the E layer hasempirically been determinedto be F, = 0.9 [(lSO + 1.44 R ) COS y]lI4 (6) where R is the sun spot number and y is the solar zenith angle A plot of the E layer critical frequency as a function of the solar angle is shown in Fig The spread due to seasonal changes and time of day is seen to be small Generally, a lower critical frequency prevails during the winter and summer The variation, as a function of solar activity, is depictedin Fig The change is even less pronounced When the short term behavior of the E region is examined, there appearchanges in theorder of 10 percent in the critical frequency which can be correlated with variation in solar output Magnetic storms not materially affect the E region Short-haul circuit requirements for daytime operation, based upon E layer reflection, can consequently be easily satisfied The F1 layer, as shown in Fig 2, is not always sharply defined Its existence is most prominent during those times when the critical F2 frequency islow, as for example, during the minimum of the sun spotcycle It is evident duringthe summerand also duringionosphericstorms There are small changes in the F1 critical frequency as a function of day to daychanges in solar activity The magnitude of the 1966 771 GOLDBERG: MF/HF COMMUNICATION SYSTEMS I I " L I 0./ O.3 0.1 COS o.+ OS o i c.7 a8 a9 I o SOLAR ZENIT# ANGLE Fig Critical E layer, frequency vs solar angle I L O 50 loo ~McWTUIZD&lNSPOr Fig IS0 tar Fig F layer critical frequency vs time of day I 250 NUMBER Critical E layer, frequency vs sunspot number change is approximately the same as that experienced in the E layer This layer, as adefined entity, exists only in the daytime The FZlayer isthe layer with the highest ionizationlevel and plays a dominant role in long-distance communication This layer is quite complex in its behavior The Fz layer critical frequency is not directly related to the solar zenith angle This layer exhibits what is known as anomalous behavior That is, it acts at times contrary to those theories useful in explaining D, E, and F, layer behavior This erratic behavior occurs during the daytime in the winter and has been labeled the winter anomaly Figure depicts idealized typical Fz layer critical frequency behavior as a function of time of day andyear Figure shows the dependence of the Fz layer critical frequency on the sun spot number The Fz layer shows a direct dependence onthe level of solar activity During sun spot maximum, the seasonal differences are enhanced The idealized MUF for various distances using Fz layer single hop propagation as a function of time of day, season, and sun spotmaximum and minimum are shown in Figs and The winter anomaly is quite evident The effects of changes in latitude and longitude on the determination of critical frequencies relate essentially t o the change in solar zenith angle at the geographical point under consideration It is fairlyevident that onelimitationto the useful transfer of information from one point to another can be expressed in terms of a signal-to-noise ratio It is for this Fig F layer critical frequency vs sunspot number o C LOCAL NODW TIME Ar I8 PATH CENTER Fig MUF vs time of day for winter NO"* LOCALf i M E A T PATH 18 c€NT€R Fig M U F vs time of day for summer I Lt 772 IEEE TRANShCl’IONS ON COMMUNICATION TECHNOLOGY DECEMBER Fig 10 Signal propagation by E and F layer sllpport 0.I I /O /oo Mcs take off angles The particular layer involved in propagation support is utilized in defining the mode of propagation For example, a single reflection from the F layer would reason that noise, as a primary factor in communication be known as the F mode, a double reflection from the E link design, is important in its own right The following dis- layer would be known as E mode cussion will be limited to what is known as external radio The transmission distance limit for single hop reflection noise and specifically, relates to atmospheric, extrausing ionospheric layers, based upon geometrical consideraterrestrial and man-made noise Figure shows the rela- tions, is dependent upon the height of the particular layer being employed For E layer propagation, this limit is in tive levels of these three noises Atmospheric noise generally consists of short pulses of the order of 2000 km; for F layer transmission, the distance high amplitude with random occurrence superimposed on limit for one hop support is about 4000 km a lower level of random noise The average value over a It is possible, and in practice happens often enough, that period of a few minutes is used to develop an averagefor a more than one path is available for the propagation supgiven hour These values are generally constant for the port of the transmitted signal It is obvious that the time hour except during local thunderstorm activity or iono- taken by each path is different, hence the signals arriving sphericsunrise or sunset The diurnalvariationin the at the receiver a t a particular instant will represent differhourly median is related to the changing propagation con- ent instantaneous transmission epochs This phenomenon ditions and the thunderstorm activity Generally, akmos- is known as nlultipath propagation andgives rise to one of pheric noise is greatest a t low frequencies, becoming rela- the major sources of trouble in long-distance communicationby highfrequencyradio The technique of transtively unimportant above 30 R’II3z mission circuit design, based upon the concepts of maxiThe extra-terrestrial noise may come from the sun, stars, mum useable frequency and frequency of optimum traffic and interstellar space Solar flares, when they occur, can D cause considerableincreases in thenoise level This galactic is predicted uponminimizing the multipath support and noise becomes greater than the atmospheric noise in the layer absorption The spread in arrival time of the transmitted signal for circuits of 3000 to 5000 miles could be in frequency region above 10 MHz overlapping signals In the HI? band, man-made noise can be a most signifi- the order of to ms These cant factor in the total noise contribution This fact pin- generate destructive interference to the composite signal points the need for proper siting when setting up a re- applied to the receiver This, of course, will decrease the ceiving location This noise is generated by any and all intelligibility of voice transmission and will create errors electrical equipment Generally, man-made noise is propa- in digital transmission Both theoretical and experimental gated by power lines and byground wave, consequently,it work has shown that multipath is a maximum a t transis notaffected by ionospheric conditions The level of man- mission path distances of about 2000 km Since the dlfferential path delay that could be tolerated made noise is highly correlated wi,ththe population density of the surrounding area Man-nmde noise may be random, is dependent upon the natureof the communication signal periodic, or a combination of both, depending on the noise and the rate of its transmission, it is important that opsources It is interesting to observethat for HF radio com- erating frequencies be chosen in order not to exceed the munication, the front endreceiver noise (internal noise) is acceptable delay Figure 11 shows the multipathreduction not the limiting factor inperformance The simple process factor as a function of path distance with the time delay of connecting an antenna toa HF’ radio receiver introduces as a parameter This factor is to be applied to the maxithe path under noise a t a level considerably higher than that developed mum useablefrequencydeterminedfor consideration by thereceiver When considering those factors affecting transmission Based upon ray tracing concepts, it is possible to define the mechanism of electromagnetil: energy transfer between reliability, in a sense, the phenomenon of multipath propathe transmitter and receiver by simply extending direc- gation could in itself be viewed as a factor in creating turtional lines to thereflecting ionospheric layer with anangle bulence in thetransmission channel However,the broader off the horizon equal to the propagation take off angle meaning of turbulence is related to solar flares, magnetic Figure 10 shows this technique for a number of different storms, and suddenionospheric disturbances (SID) Fig Noise level as a function of frequency 1966 S Y S T ECMOSM M GOLDBERG: U N I C A T I O NM F / H F 773 a single, normal, sharply defined vertically sounded return frequency vs layerheight,manyheightsappear,hence there is multiple support for each frequency.The effect on signals propagated via Spread F is generally to introduce upon the signal rapid fluctuations characteristic of scatter communications For the third disturbance within thegeneral framework 10' IO ' IO ' O I ' of turbulence, it is appropriate to discuss fading There are DISTANCE, K W various types of amplitudefading.Theseare generally relatedtothe period betweenminimums The shortest Fig 11 H F multipath reductionfactors interval generally relates to polarization fading between the ordinary and extraordinary waves andis known as interference fading Periods ranging from about 0.1 seconds The solar flare usually lasts less than one hour andgen- to a few minutes are responsible for both selective and flat fading Selective fades relate to specific frequencies within erally occurs most frequently during sun spot maximum while flat fades relateto the During the flare, large amounts of ultraviolet and X-rays a transmissionband fading out entire band fading out When the fade periods are approxiare emitted which, in turn, cause large increases in the D mately five minutes or more, the fades are generally layer electron density This has theeffect of increasing the attributable to absorption changes in the D layer absorption of electromagnetic energy passing through the The distribution of the amplitude of the signal envelope region and of significantly decreasing signal strength The for the common type of fading (both selective and flat) immediate effect on theE and F layers appears to be small which is generally due to multimode support of propagaHowever, since the energy for earth-bound stations must tion, is bestdescribed by the resultantof a composite wave pass throughthe D region a t least twice, the effect on commade up of a Rayleigh distributed amplitude and a steady munication is already felt The SID or short wave fadecomponent This type of distribution is known as a Rice outs (SWF) last a relatively short time from minutes to distribution This distribution has the attribute that when hours and are experienced in the sunlit regions of the Earth the specular component is small, the distribution is essenThe magnetic disturbances are usually experienced about when the specular component is 20 to 40 hours after the onset of a flare This is attributable tially a Rayleigh type and large, the distribution is essentially Gaussian to the lower energy corpuscular radiation from the flare It is interesting to note that most of the information The magnetic effects last from two to five days It is this relating to the critical frequencies for each layer, the level delayed effect that is the most troublesome The magnetic of electron density, the existence of Sporadic E and Spread and ionospheric stormsare a worldwide phenomenon which, in severe cases, affect practically all high-frequency F, and the general state of the ionosphere is obtained by means of electromagnetic probing using a device known as transmission employing the ionosphere During the sun spot maximumthe turbulence is more severe, but of shorter an ionospheric sounder In thelast few years, however, this duration (two days), while during sun spot minimum, al- has been supplemented by rocket and satellite sounding from both sides of the ionosphere though the flare occurrence is rarer, its effects last for a Up to themost recent time, the technique was to launch longer time-five days The most significant effect of an ionospheric storm is the a vertically directed wave (pulse) and monitor its return reduction of the Fz layer critical frequency.I n addition, the on an oscilloscope By measuring the time delay forthe return over a bandof frequencies, it is possible to develop an F layer acts more like a diffuse-scattering surface rather than a surfacethat provides reasonably specularreflection electron densityprofile using the mathematical relationship The effects of F layer electron density reductionare greater between the criticalfrequency and the electrondensity at higher geomagnetic latitudes It ishas been noted noted in (4) More importantly, for communication, it is that during sun spot minimum there appears to be a 27- possible to observe the critical frequency for each layer day cycle tothe ionospheric disturbance.This period directly Considerable skill is required in order to interpret the results and much manual processing is needed corresponds to the solar period of rotation I n addition to the disturbancescorrelatedwith solar Figure 12 shows an idealized return from a vertical ionoflares, a t least three other phenomena are common causes spheric sounder An ionogram such as this is quite rare, of transmission turbulence The first of these is known as most of the time there isconsiderable interference present Sporadic E These ionization clouds located in theE layer and each return is made upof two lines due to theeffect of region support the propagation of electromagnetic energy the Earth's magnetic field splitting the electromagnetic a t frequencies considerablyabove the normal E layer wave into two differently polarized waves, because each is RIIUF The ionization clouds travel, and in numerous in- reflected by a different electron density These two waves stances have provided E layer height propagation support are known asthe ordinary and extraordinary rays Ionospheric sounders are located in field sites all around a t night, when the normal E layer is absent A second disturbance known as Spread F is manifested the world The job of these stations isto collect data using as a continuum of F layer height in that rather than having15-minute intervals regarding the critical frequencies for 774 COMMUNICATION DECEMBER TECHNOLOGY ON TRANSACTIONS IEEE 1) Estimatethegreat circle distancebetweentransmitter and receiver site and locate its midpoint, in terms of its geographic coordinates 2) Determine midpoint local time 3) Determine MUF from predictioncharts for particular zone of interest as a function of time of day and midpoint geographical location 4) Plot these pointsfor a full24 hours 5) The optimum working frequency is then taken as 85 percent of these values 1000 900 aoo 700 600 500 c - 400- I Y I 300 200 - E LAVER 100' Another curve must be developed in order to define a lower limit to the choice of frequencies available a t a parFREPUENCY mc/s ticular time This known is asthe LUFor lowest useful freFig 12 Vertical sounder ionogram quency Thislimiting frequencyis determined by the signal strength required at thereceiving location This, in turn,is related to the local noise level which sets the threshold the various ionospheric layers The information from the against which the desired signal-to-noise ratio is established various stations are collated at the Bureau of Standards for the required performance criterion The received signal and from this data, world charts are developed showing strength is, of course, related to the transmitter power involved, the transmission the critical frequency as a function of time of day and available, antennasystems distance involved, and the absorption losses experienced by geographical locations This information is then used to the electromagnetic wave The signal strength determined form predictions about the R4UF' and FOT thatshould be in this manner is highly dependent upon the frequency a t used for particular communication paths which it has been calculated It is necessary to determine A recent development in the ionospheric sounding art the lowest frequency a t which the required signal strength is the oblique sounder.This device permits ionogramsto be will be achieved A plot of these values for diff erent times madeusing the actual communication path that would of day will then be the locus of the lower limit of useful frenormally be employed for traffic The technique is to use a quencies for the path under consideration in terms of the stepped frequency transmitter for sendingthe probing sigthatthe signal nal, while at the distant receiving site the receiving system type of service required It isnoted strength requiredis significantly related to the type of islocked instepwiththetransmitter.Thistechniqueactually permits the measurement of communication sup- modulation employed and the reliability required Figure 13 shows the result of a determination of the port to be determined a t will &lore importantly, i t frees the communicator from reliance on predictions which are FOT and LUF for different length circuits The idea for not always reliable The potential for optimizedcommuni- circuit operation is to choose operating frequencies which cations frequency determination on a real-time basis is a t fall within the bounds of FOT and LUF I n general, the hand through the use of oblique sounders This technique complexities of propagation coupled with theneed for relying on predictions that are theresult of many approximais just beginning to develop tions makes the choice of an optimum operatingfrequency Utilization of most of the preceding informationin terms of establishing the interplay between the various dif€icult However, some broad guidelines are possible A factors considered for engineering long-haul ionospheric useful and obvious criteria is to adjust things so that an transmission networks can best be established by employ- adequate signal-to-noise ratio is achieved The step to be ing prediction charts issued by the Bureau of Standards taken in thisdirection is to use the highest frequency that Although the procedure is well laid out, theresult achieved will propagate to the distant receiver This pays off, since makes the employment of this approach an art rather thanradio noise decreases as the frequency is raised while absorption is likewise decreased The use of frequencies a science The usualprocedureis to employ prediction charts near the MUF, in addition,results in less likelihood of issued by theBureau of Standard.s everymonth which pre- multipathprogagation The art of engineering high-frequency communication dicts critical frequencies three months in advance for all parts of the world These predictions are updated by means circuits is well documented by myriad publications from the National Bureauof Standards and the Radio Propag* of a monthly, weekly, daily, and evenhourlyadvisory issued by NBS An important factor used in developing tion Agency of the U S Army We now reach the point where we can discuss what we such charts is the solar activity index An incorrect estimateinthisfactor would came troublesomeerrors in have learned about theHF ionospheric mode of communication Equipments utilizing this modeemploy voice, estimating MUFs for particular circuits The techniqueinabbreviatedform is essentially as music, TTY, facsimile, data, andeven noise as modulation sources With proper conditions, these equipments can be follows: s 1966 GOLDBERG: MF/HF COMMUNICATION SYSTEMS 775 Fig 13 FOT-LUF prediction used for essentially around the world communications with space and the already dense packing of users in this pordistances being established by choice of operating fre- tion of the radio frequency spectrum, assignments are not made that broad quency, radiated power, and antennatake-off angle I n general, 12 kHz of RF spectrum spaceis about as large Ionospheric propagation, as has been indicated, is gengenerally erallycharacterizedbymultiplehop ionospheric layer- a slice that can beassigned In themilitary this is ground reflection withboth specularandrandom com- utilized to carry four kHz channels of information conponents of energy arriving atthe receiving antenna sisting of either voice, TTY, facsimile, or data The most critical and fundamentalsignal function that This energy, because of the time variant dispersive propercould be used to characterize the various forms of modulaties of the ionospheric medium, occupies a fading bandwidthfrom 0.05 to 15 Hz dependingupon the level of tion is considered to be a digital signal Ultimately, i t is turbulence Nonauroral path propagation generally has an expected that all information will be handled on a digital upper limit of about Hz The envelope of the composite basis With this thought in mind, the USAEL undertook received signal exhibits Rician statistics withthe Rayleigh a program to measure the properties of the ionospheric statistic subset predominating as indicated earlier Limited channel in terms of its fine grain behavior in both phase of the actual transmission data relating to measurement of the correlation bandwidth and amplitude and in terms of the work, both FSIi indicate that it varies from about 100 to 3000 cycles de- of digital signals For the latter part pending upon the channel turbulence The time spread of and PSI< were employed arriving energy varies from less than 100 ms to about4 ms A series of examples of the phase stability of the ionoWith good pathsand properoperating frequencies the spheric medium for various averaging times canbe seen in multipath spreads areless than 1ms I n this connection, it Figs 14-16, where we can see the effects on measuring is noted that ionospheric propagation via an auroral path phase (such as a PSI< system must do) of a decreasing is generally much more turbulent than nonauroral trans- signal-to-noise level A bottoming effect on error rate is mission It is quite possible that the fade rate may be as apparent It is also clear that in thepresence of high signalhigh as 25 Hz while the correlation bandwidth may be to-noise ratios higher reliabilities appear possible in PSK as narrow as50 Hz or less over an auroral path I n general, systems if the bit length is decreased (shorter averaging the ionospheric channel is limited in performance by both time) I n Figs 17-19 we see, for a particular averaging timea t additive disturbances such as atomspheric noise, friendly interference, and basic propagation loss factorsandby approximately the same signal-to-noise ratio, the effects of multiplicative effects such as faderate and the Doppler and increases in channel turbulence time spread of the received energy I n order to account for the measured phase behavior By applying effective techniquessuch as space, fre- shown in Fig 14, Fig 20, representing the theoretical quency, or time diversity reception and the proper choice curves for Rayleigh fading at an infinite signal-to-noise of operating frequency, it is possible to have better thana ratio, with the product of fading bandwidth and averaging time as parameter, is presented The similarity isobvious 90 percent reliability factor forthis typeof channel The fading bandwidth of a received CW signal can be Although R F bandwidths of up to 20 kHz (under good conditions) can be adequately supported by this medium, expressed in terms of the envelope (for a Rayleigh fading it is noted that because of the hiah demand forassignment, pIC.4 Fig 14 Phasestability vs weraging time, high Fig 15 Phase stability vs averaging time, medium SNR SNR Fig 16 Phasestability vs averaging time, Fig 17 Phase stability, low SNR low fading rate 20 Fig 18 Phase stability, medium fading rate 60 LO 80 Fig 20 Theoretical phase FB2 where R = loo 120 a0 stability for Rayleigh fading = R - 27r2 R2 envelope of signal Using the theoretical curves in Fig.20, it hasbeen possible to extract from measured phase curves such as shown inFigs 14-19, estimates of the fadingbandwidth The distributions for two classes of runs are shown in Fig 21 It shouldbenoted thatthefading bandwidth is not the bandwidth of the power densityspectrum.For instance, with a rectangularly shaped power density spectrum of width B, sayone cycle, the fadingbandwidth would only be: FB Fig 19 Phase stxbility, high fading rate = B FS ~ 0.288 (8) The short-termamplitudecharacteristicsare also of significance Figures 22 and 23 show the distributions for fine grainsignalamplitudemeasurementsfor mild and sever'e conditions with superimposed Rayleigh theoretical curves From a relatively large collection of data such as this and the phase data cited earlier, it seems justified to employ the statistics of narrow-band Gaussian noise as the model of the time variant dispersive effect on ionospheric transmission of signals The distribution of fading periods for mild' and severe conditions can be observed in Figs 24 and 2.5 It is apparent that as the level of turbulence is increased, the fadingperiod is decreased 778 IEEE TRANSACTIONS COMMUNICATION TECHNOLOGY ON DECEMBER J FADING PERIOD SECONDS 54- 2.0 1 - I o 45 LI, I1 I *I5 ,i LC ! FADING PERIOD Fig 24 30 4.0 SECONDS 5.0 Distribution of the signal fading period (mild conditions) FB I 35 (b) Fig 21 Distributions of fading brmdwidths (a) Low-frequency group to 11 MHz, total number of 22 runs: 25 (b) Highfrequency group 17 to 21 MHz, total number of 22 runs: 33 iAOlNC PERIOD SECONOS Fig 25 ulcRovotTs INPUT sII;nAt Fig 22 Distribution of average input signal amplitude (mild conditions) MICROVOLTS INPUl SIGNAL Fig 23 Distribution of average input signal amplitude (severe conditions) Distribution of the signal fading period (severe conditions) Figure 26 showsa distribution of fadedurationsfor variousthreshold crossings below a6-secondaverage We can see, for example, that if we were concerned about fades of 20 dB below the 6-second average (which could have somewherebetween 35 and 50 dB signal-to-noise ratio), there will be a probabilityof approximately 10 percent that the fadewill last a t least 300 ps It would appear that about 22 bits at a 75 bits per second signaling rate would be clobbered during this time However, it must be noted that this mould not be the case, since, during this fade interval, in general, only the central bitswould have been exposed to instantaneous signal-to-noise ratios low enough to cause the bit error rate to reach 0.5 The other bits in the interval would have probabilitiesof error related totheirinstantaneousbit signal-to-noise ratio As an estimate of what would have happened during this interval, it is judged that it mould be unlikely for more than bits out of the 22 to be in error Thesemeasurementsweremadeover the Hawaii to Deal, N J., path, which isbasic,allyone of our better paths 1966 779 GOLDBERG: MF/HF COMMUNICATION SYSTEMS 10 aeo Fig 26 Tape 68 Track Recorded February 23, 1963 Time 1600 EST Distributions of the time duration of a fade Carrier frequency 20425 kHz Average SNR 49 dB 60 signal Average pV Average fading rate 0.126 Hz Information such as this is basic to the ultimate design of effective coding for H F ionospheric channel Over the years, it has been convenient for USAEL to categorize the state of ionospheric turbulence during tests of communication equipment Figure 27 shows this classification Generally, conditionson a circuitare such that the principal diagonal (left top to right bottom) receive the greatest number of data samples As a qualitative classification the left column can beconsidered as representative of mild, the middle column medium, and the rightcolumn severe propagation conditions Figure 28 shows examples of performance of an FSKsystem and a DPSK system under mild and severe propagation conditions These tests were conducted using space diversity reception with the FSK (AN/FGC-29) system operating a t 1200 b/s and the DPSK system (AN/FGC-54) operating a t 3000 b/s Bothsystems were operated a t equal power per system each using approximately a kHz portion of the RF spectrum Fade thresholds3 dB l O d B dB 20 d B - Threshold lncreasmg right to left 1-2 0-2 1-3 B-3 02 03 Fig 27 Propagationcategories The significant points to be made are that we see the existence of bracketing regions of performance The existence of asymptotes to performance of FSK and PSI< systemshas beenmeasured for some time now with the implication that finiteincreases in power would be in- 780 IEEE TRANSACTIONS ON COMMUNICATION TECHNOLOGY DECEMBER 10 lo' s 10- s h n \', rn \ lo-' c, 10 Fig 28 Comparison ofFSK and PSK system performance effective in overcoming the loss in digital data reliability width factor without considering the impact of multipath duetothetimevariant dispersiveproperties of the propagation and its additional large contribution to the medium I n this connection, Voelcker, i\/lasonson, and Bello irreducibleerror rate resultingfrom the generation of havemade significant contributionstothetheory sup- interchannel crosstalk andloss of signal set orthogonality porting these observations I n Figs 31 and 32, we see examples of a lower bound of During the last few years the underlying analytic mecha- performance due principally to atmospheric noise Here we nism capable of accounting for the measured performance see excellent agreementbetween the measuredresults, of FSK and PSI< systems have evolved with theresult that under mild and reasonably nonperturbed conditions where useful and reliable predictions can now be made with re- atmospheric noise would be expected to limitperformance, gard to system performance under dispersive channel con- and the theoretical predictions for an FSK and a DPSK ditions digital data system under atmosphericnoise conditions Examples of such theoretical results for dispersive media We now feel that withourpresentunderstanding of are seen in Figs 29 and 30 where the bottoming effect is ionospheric transmission that system performance can be quite evident It should be noted that theseresults are for predicted quite closely once certain basic information nondiversity operation An appropriate shift in scale would relating to the turbulence of the ionospheric channelis be required to utilize these curves for diversity reception; known the shape of the curves would not change It must be It appears that digital errors under high average SNR pointed out that these results are based upon consideration are bounded a t their lower error rate bound byatmospheric of turbulencein the channelthrough the fadingband(non-Gaussian) noise under nonperturbed conditions and 1966 I 781 SYSTEMS COMMUNICATION GOLDBERG: HF/MF /\\ DPSK error-rate = 1 2r2/FB2/BR2 + M + m - ~ in slow fading: - 21+M' ~ FB=O' ~ 1 -2 - + M = (BR2/20FB2) M' in fading limited region: 10 FB2/BR2,M = F B = fading bandwidth, RR = bit rate = / T &I = mean signal power-to-noise power at cross-over: FSE; error-rate = Fig 29 Theoretical PSK bit error rate vs SNR and fading band-width a t their higher error rate bound, by an irreducible error rate dependent upon the time and Doppler spread (fading bandwidth) I n order to obtain a data base relating to ionospheric channels that mould permit more precise estimates to be made of system performance, USAEL has undertaken a field test program with contractual help to measure and then generate the following information in terms of the diurnal variations on the measured values and the choice of operatingfrequency and ionospheric support mode: 1) autocorrelation of phase angle 2) cross correlation between frequency spaced received signals 3) probability density of received signal 4) autocorrelation of received signal ) probabilitydensity of signal envelope 6) autocorrelation of signal envelope 7) probability density of phase angle between different tones 8) cross correlation between envelopes of diff erent tones 9) cross correlation between phases of different tones 10) cross correlationbetween envelope and phase of same tone 11) fading bandwidth 1 in slow fading: -2 { ( l + & ) ( l + & ~ ) , F B = i n fading limited region: ( F B / D ) ,approx ( M = m) Fig 30 Theoretical FSK bit error rate vs SNR and fading band-width 12) 13) 14) 15) coherent factor time spread frequencyspread bit error rate We expect that this information willgo a long way toward removing the need for speculation about the basic behavior of the medium and permit substitution for this speculation the quantitative values obtained from measurement The fundamentalpurpose of all this effort and that described earlier is related to specific needs of the military One use would be to permit the measurement of certain critical parameters in real time so that predictions of system performance in real time canbe made without the need to actually examine the received digital data A second and more significant use would be to employ the results to guide the development of optimum data 782 TRANSACTIONS IEEE ON COMMUNICATIONDECEMBER TECHNOLOGY SYSTEMSNR SYSTEM SNR(GENERALDYNAMICS I GENERAL DYNAMICS TEST1 - TEST 1- Fig 31 Comparison of theoretical and measured FSK performance in atmospheric noise Fig 32 Comparison of theoretical and measured PSK performance in atmospheric noise completely terminals for the dispersive ionospheric medium I n this using shortbauds.Theentireoperationis channelprobe signals occurringoften connection, the USAEL has alreadydeveloped, through its automaticwith enough to follow the time variantbehavior of the channel supportingcontractors,twos,ystems designed to match The adaptive approach opens up a new concept in HF the changing data rate support of the perturbed ionospheric communications in that data under the proper conditions channel These systems fallinto thecategory of what we call self- may possibly be sent over a kHz channel at the rate of automated adaptive comnmnication terminals responsive 4800 to 9600 b/s whereas before, serious problems developed when we a.ttempted tosend 2400 b/s at anacceptto the data rate support of the :medium One system already field tested, is known as the AN/ able error rate level The use of short bauds intransmission over a dispersive GSC-10 It employs RAKE principles and reference tracking in addition to sophisticated processing known to be HF ionospheric medium represents a majordeparture from effective when designed specifically for time variant dis- the heretofore accepted practice In fact,full exploitation persivetransmissionchannels A much simplified block of this concept requires basic data about the transmission medium in terms of short baud transmission which is, a t diagram of the system is shownin Fig 33 The second approach to adaptive communication now this time, very scarce We expect to be adding to the data being fabricated, called ADAPTICOM, employs the means base in this areaalso in the near future The full ramfications of the adaptive approach tocomfor measuring the transfer function of the perturbed medium Thisinformation is utilized at thereceiving terminal munications have many usefulside effects For example, in existence of multiple to create a matched filter tothe medium and then operate theADAPTICOMapproachthe on the outputof the matched filter to reduce the side lobe paths of propagation is actually employed as sources of diversity input which are processed so as to provide coresponse of its essentially sinx/z output This system in herent gain in the equipment I n this way, it appears that simplified form is shown in Fig 34 a more optimum approach to a choice of operating freBasically, the communication concept istointerlace probe signals with the data to be sent The probe signal quency is away from the RIUF toward the ordinarily undesired, henceunused by other communicators, part of sets up the receiving networlts so as to make the total the spectrum Two advantagescould accrue from this fact, transfer function fromthe transmitter antenna through the receiver terminal appear identical to that of a lossy linear one is that the available spectrum for communications is broadened and, two, there would be less mutualinterphase, constant time delay, nondispersive network Once the receiving networks are set data is transmitted serially ference 1966 GOLDBERG: M F / H F C O M M U N I C A T I O N 783 SYSTEMS V (b ) ~ Fig 33 Simplified block diagram AN/GSC-10 system (a) Transmit terminal (b) Receive terminal r- Y - - - - t - , a- Fig 34 Simplified block diagram ANFYC-5 system (a) ADAPTICOMtransmit receive terminal terminal (b) ADAPTICOM VOL C O M - ~ ~NO , IEEE TItANShC‘lTONS ON COMMUNICP.TION TECHNOLOGY DECEMBER 1966 [3] “Ionospheric radio propagation,” Nat’l Bur Std Circular 462, 1943 theZonosphere Cambridge, [4] K G Budden, RadioWavesin England: Cambridge University Press, 1941 [5] S K Mitra, TheUpperAtmosphere Calcutta,India: Asiatic Society [6] ‘:Basic radio propagation predictions,” Nat’l Bur Sld., CRPL Series D [7] “Reference data for radio engineers,” ITT, 1956 [8] Nat’l Bur Std RadioPropagation Course Notes, 1961-1962 [9] B Goldberg, “HF radio data transmission,” I R E Trans on Communications Systems, vol CS-9, pp 21-28, March 1961 [lo] “Evaluation of a new high frequencyradiocommunication equipment,” General Dynamics Corp., Final ltept.2 for task 1, Rept AS 272 565, 1961 [11] ‘Wtudy of fine grain fading and phase stability of multiple CW signals,” General Dynamics Corp., Rept for task 3, Rept AD 406 213, 1962, [12] “Study of fine gram fading and phase stability of multiple CW signals,” General Dynamics Corp., Rept for task 3, Contract DA 36-039 SC-88943, 1963 [13] J Korte and C Jackson, “Evaluation of high frequency communicationsequipment using frequency stabilized receiver,” UASELRDL, Test Rept 1544 June 1963 [14] J F Korte and J S, Koch, “Measurement of the phase perturbations of a CW slgnal over a long haul H F circuit and its comparison withanalytical resultsfor a Rayleighfading signal,” (Addendum to 1111 and 1121) USAEL Rept., April 1, 1964 [15] “Ionospheric transmission models, task correlation between transmission parameters of dispersive circuits andsystem performance for application to adaptive communications systems,” RCA Defense Electronic Products,ContractDA 36-039 SC-87240 [IS] B Goldberg, L B Shucavage, and J Korte, “Fine grain ionospheric behavior,” Globecom VZ Symposium Digest, Philadelphia, Pa., June 2-4, 1964 [17] ‘Characterization of radio channels,” Adcom Inc., Interim Rept Contract AD 28-043 AMC-00038 (E), 1964 [18] "Analytical and experimental study of correlatlon function Communications Systems Inc., Final over HF circuits,” Rept., Contract DA 2S-043-AMC-O0145(E), 1965 CONCLUSION At the present time we ha1.e a good handle on the control of the dispersive properties of t h e H F medium The possibility exists that the use of adaptivesystems will permiterrorratestoberesponsive toandimprovable upon by increases in received signal-to-noise ratio without having to cope with a high ir.reducible error rate It is expected thak the remaining PI-oblem of atmospheric noise will be ov’ercome by means of effective coding We feel that in the near future HI? ionospherictransmission of digital data will attain a level of reliability a few orders of magnitude beyond present capabilities reaching a state of performance thought impossible just a few years ago ACKNOWLEDGMENT The author would like to a,cknowledgethe fine support given to the USAEL general program in HF communication research by GeneralDynamics,Inc.(StrombergCarlson Division), RCA Inc., Defense Electronic Products, Adcom Inc., and Communicstions Systems Inc He also acknowledges thecontributionsand assistance of L B Shucavage and J Korte, both of USAEL, in the conduct of the many programs that gave rise to the datapresented REFER.ENCES [I] “Radio propagation,” Department of the Army, Rept TRI 11499, 1950 [2] F E Termon, Radio Engineering Handbook New York: McGraw-Hill, 1943 Optimum 13inary FSK for Transmitted Reference Systems Over Rayleigh Fading Channels Absfracf-It is well known that in communicating over randomly time-varying channels, a receiverwhich performs a channelmeasurement can make a better decision than one that does not Furthermore, if the channel characteristics vary relatively slowly in comparison to a large number of adjacent message intervals, a small portion of the transmittter energy can be devoted to channel measurement, and, in spiteof the loss of energy in the information bearing portion of the signal, the resulting system performs better than one with no measurement This p,aper shows that improved system performance from a channel measuring system occurs, even when the channel characteristics are fixed only during the presentmessage interval The randomly time-varying cha.nne1studied is that of a Rayleigh fading medium with independently fading mark and space channels whose fading is fixed over one haud interval but is independent Manuscript received June 4,19fi5 The author is with the University of California, Imine, ‘Calif He was formerly with the Air Force Cambridge Research Laboratories, Bedford, Mass from baudto baud The transmission systemis a modified frequency shift keying (FSK)system such that during a portion of a baudinterval, the mark and space frequencies are always transmitted so a s to act as reference signals For this system, the following has been established: 1) optimum receiver configuration ) optimum ratio 01 of information energy to total signal energy as a function of total available SNR for a single fading channel 3) asymptotic optimum 01 for an M-diversity channel ) error probabilities for item and asymptotic error probabilities for item for ooptas a function of total SNR The asymptotic results show that by using reference techniques the order of diversity is effectively doubled INTRODUCTION I 784 NFORMATIONtransmissionoverrandondytimevarying channels has been studied by many authors [1]-[3] Kailath has studied the Gaussian, randomly time- [...]... PERIOD SECONDS 54- 2.0 3 1 1 - I 1 o 0 45 LI, I1 I *I5 ,i LC ! 3 FADING PERIOD Fig 24 30 4.0 SECONDS 5.0 Distribution of the signal fading period (mild conditions) 3 FB 2 I 35 (b) Fig 21 Distributions of fading brmdwidths (a) Low-frequency group 8 to 11 MHz, total number of 22 min runs: 25 (b) Highfrequency group 17 to 21 MHz, total number of 22 min runs: 33 iAOlNC PERIOD SECONOS Fig 25 ulcRovotTs INPUT... Thesemeasurementsweremadeover the Hawaii to Deal, N J., path, which isbasic,allyone of our better paths 1966 779 GOLDBERG: MF/ HF COMMUNICATION SYSTEMS 10 aeo Fig 26 Tape 68 Track 3 Recorded February 23, 1963 Time 1600 EST Distributions of the time duration of a fade Carrier frequency 20425 kHz Average SNR 49 dB 60 signal Average pV Average fading rate 0.126 Hz Information such as this is basic to the ultimate... These tests were conducted using space diversity reception with the FSK (AN/FGC-29) system operating a t 1200 b/s and the DPSK system (AN/FGC-54) operating a t 300 0 b/s Bothsystems were operated a t equal power per system each using approximately a 3 kHz portion of the RF spectrum Fade thresholds3 dB 0 l O d B 0 6 dB 20 d B - 0 Threshold lncreasmg right to left 1-2 0-2 1-3 B-3 02 03 Fig 27 Propagationcategories... fades of 20 dB below the 6-second average (which could have somewherebetween 35 and 50 dB signal-to-noise ratio), there will be a probabilityof approximately 10 percent that the fadewill last a t least 300 ps It would appear that about 22 bits at a 75 bits per second signaling rate would be clobbered during this time However, it must be noted that this mould not be the case, since, during this fade interval,... bounded a t their lower error rate bound byatmospheric of turbulencein the channelthrough the fadingband(non-Gaussian) noise under nonperturbed conditions and 1966 I 781 SYSTEMS COMMUNICATION GOLDBERG: HF /MF /\\ DPSK error-rate = 1 1 2r2/FB2/BR2 2 1 + M + m - ~ in slow fading: - 21+M' ~ FB=O' ~ 1 1 -2 2 - + M = (BR2/20FB2) 1 M' in fading limited region: 10 FB2/BR2,M = F B = fading bandwidth, RR = bit... supportingcontractors,twos,ystems designed to match The adaptive approach opens up a new concept in HF the changing data rate support of the perturbed ionospheric communications in that data under the proper conditions channel These systems fallinto thecategory of what we call self- may possibly be sent over a 3 kHz channel at the rate of automated adaptive comnmnication terminals responsive 4800 to 9600... communication now this time, very scarce We expect to be adding to the data being fabricated, called ADAPTICOM, employs the means base in this areaalso in the near future The full ramfications of the adaptive approach tocomfor measuring the transfer function of the perturbed medium Thisinformation is utilized at thereceiving terminal munications have many usefulside effects For example, in existence... and a DPSK ditions digital data system under atmosphericnoise conditions Examples of such theoretical results for dispersive media We now feel that withourpresentunderstanding of are seen in Figs 29 and 30 where the bottoming effect is ionospheric transmission that system performance can be quite evident It should be noted that theseresults are for predicted quite closely once certain basic information... known as the AN/ able error rate level The use of short bauds intransmission over a dispersive GSC-10 It employs RAKE principles and reference tracking in addition to sophisticated processing known to be HF ionospheric medium represents a majordeparture from effective when designed specifically for time variant dis- the heretofore accepted practice In fact,full exploitation persivetransmissionchannels... cross correlationbetween envelope and phase of same tone 11) fading bandwidth 1 1 1 in slow fading: 2 -2 { ( l + & ) ( l + & ~ ) , F B = 0 i n fading limited region: ( F B / D ) 2 ,approx ( M = m) Fig 30 Theoretical FSK bit error rate vs SNR and fading band-width 12) 13) 14) 15) coherent factor time spread frequencyspread bit error rate We expect that this information willgo a long way toward removing ... levelin the highfrequency 3 -30 MHzband This portion of the spectrum is highly crowded as is the I n general, communications reliability in this region tends entire range 0.3 -30 MHz Although the... merges with the F2 layer a t nighttime to a height of about 300 km The electron densityin thisregion is inthe order of lo6electrons per cc The 300 - km layer height, called the F layer, is usually considered... of vated by the ever present and generally increasing man- mediumfrequencyrange, the groundwavedistance at 1966 769 GOLDBERG: MF/ HF COMMUNICATION SYSTEMS acceptable attenuation levels is much